1. Field of the Invention
The present invention relates to a method of controlling a DC/DC converter which is suitable for use in a hybrid power supply system for supplying a load with electric power from a first power device and a second power device, and a fuel cell vehicle for carrying out such a method. In the fuel cell vehicle, for example, an inverter-driven motor serving as the load is supplied with electric power from a battery and a fuel cell.
2. Description of the Related Art
Heretofore, there has been proposed a DC/DC converter apparatus which is disposed between a high-voltage battery and a low-voltage battery for bidirectionally converting voltages (a high voltage into a low voltage and a low voltage into a high voltage) and bidirectionally passing currents (see Japanese Laid-Open Patent Publication No. 2002-112534).
There has also been proposed an apparatus including a DC/DC converter disposed between a high-voltage power supply in the form of a rectified AC power supply and a battery, for energizing a motor through an inverter under a secondary-side voltage of the DC/DC converter, i.e., the voltage of the high-voltage power supply (see International Publication No. WO 2002/093730).
According to the apparatus disclosed in International Publication No. WO 2002/093730, when the motor operates in a propulsive mode, the high-voltage power supply supplies a current to the motor, and the battery supplies a current to the motor through the DC/DC converter. When the motor operates in a regenerative mode, the battery is charged by the high-voltage power supply and the motor through the DC/DC converter. Consequently, the DC/DC converter disclosed in International Publication No. WO 2002/093730 also operates to bidirectionally convert voltages and bidirectionally pass currents.
The DC/DC converter for bidirectionally passing currents, as disclosed in Japanese Laid-Open Patent Publication No. 2002-112534 and International Publication NO. WO 2002/093730, basically comprises upper and lower arm switching devices and a reactor, and operates according to a synchronous switching scheme wherein the upper and lower arm switching devices are alternately turned on respectively before and after a dead time within one switching period. The dead time is inserted between the on-times of the upper and lower arm switching devices to prevent them from being simultaneously turned on and hence to prevent the high-voltage power supply from being short-circuited.
In the DC/DC converter which is capable of bidirectionally passing currents for synchronously switching the upper and lower arm switching devices, the reactor stores energy when the switching devices are turned on and discharges the stored energy when the switching devices are turned off. Therefore, triangular-wave currents having upper and lower peaks flow through the reactor.
The inventor of the present application has found that when the triangular-wave currents change across a zero value at which their current-flow directions are changed, the output voltage (control voltage) of the DC/DC converter develops surges (peaks) though the target voltage is constant (see FIGS. 11 and 13 of the accompanying drawings). The phenomenon of the surges will subsequently be described by way of a comparative example in connection with an embodiment of the present invention in the description of the invention for an easier understanding of the invention.
When a surge voltage is produced, a power apparatus and a load which are connected to the DC/DC converter have their efficiency lowered. Since it is necessary to establish higher settings for the withstand voltages of the power apparatus and the load and also the withstand voltages of the switching devices of the DC/DC converter, the DC/DC converter, the power apparatus, and the load have their costs increased.